The molecular basis of several human diseases has recently been found to be expansions of simple trinucleotide repeat DNA sequences. In this proposal, we focus on the neurological disease myotonic dystrophy (DM), which is caused by a (CTG)n expansion in the 3'untranslated (3'UTR) region of the DM protein kinase (DMPK) gene. The long term objectives of this proposal are to understand the cellular consequences of the CTG repeat expansion and the pathogenesis of myotonic dystrophy. The Specific Aims of this proposal are to: 1) develop and analyze a transgenic moused model for myotonic dystrophy; and 2) analyze proteins which bind to the repeat expansion in genomic DNA (CTG)n or RNA (CUG)n. The health relatedness of this project are that it will provide a better understanding of the pathogenesis of one of the commonest muscular dystrophies, myotonic dystrophy, and about diseases caused by triplet repeat expansions in general. It will also provide an animal model for DM allowing different therapies to be tested in the future. The research design and methods we plan to use are: 1) Gene targeting by homologous recombination in embryonic stem cells to target a (CTG)n repeat expansion to the 3' UTR of the mouse DMPK gene. Transgenic mice harboring and expanded CTG repeat in the DMPK gene will be analyzed for repeat stability and the pathophysiological features of DM. 2) We have previously identified, purified and partially sequenced two proteins which bind to single stranded DNA (CTG)n and RNA (CUG)n repeats. The genes encoding these proteins will be cloned and sequenced to study gene structure and chromosomal localization. Antibodies to these repeat binding proteins will be used to study the expression pattern of these proteins in normal and DM tissues and recombinant proteins will be used to study their RNA binding properties. This approach is designed to explore the function of these CTG/CUG repeat binding proteins and their possible role in the pathogenesis of DM.